Those skilled in the art will appreciate the harsh operating environment of communication devices such as mobile radios, especially in a low frequency operating bandwidth. The major contributors to a severely noisy environment for the mobile radio include engine noise, at frequency bands below 174 MHz, (both from the vehicle using the mobile radio and surrounding vehicles), electrical interference from high power lines, and atmospheric disturbances.
Some mobile radios have employed noise blankers to suppress or eliminate these noise effects thereby improving system coverage, especially in weak signal areas. The basic purpose of a noise blanker is to detect the presence of impulse-type noise and momentarily prevent the noise in the recovered signal from reaching the intermediate frequency (IF) stage. Thus, for the noise blanker to function properly, it must detect the presence of noise and inhibit the signal path in the main receiver before the noise gets to the point where it is unwanted (to be stopped.) Hence, noise blankers require a delay in the signal path to allow time for any ignition or other noise pulses to be processed and gated off before they reach the crystal filters in the IF stage where the noise pulse will be stretched in time due to ringing in the highly selective crystal filters.
Historically, implementation of a noise blanker in a mobile receiver was facilitated by the commensurate bandwidth of the main receiver and the noise blanker (i.e. each about one Megahertz) such that a "race" condition was not a significant problem. Since the bandwidths were practically the same, the delay was effectively the same or could be compensated for by small "lump element" filters.
Modern mobile radios however, have extremely broad bandwidths. Since most mobile radios have frequency synthesizers that can generate a wide variety of frequencies, mobile radios today use broad bandwidth filters permitting the mobile radio user to operate over a wide band of frequencies. Thus it is common for a receiver to have a bandwidth of twenty or thirty megahertz. However, this bandwidth extension creates significant problems in the operation of the noise blanker circuitry. Since the bandwidth of the main receiver may be twenty times the bandwidth of the noise blanker (thus making the noise blanker delay twenty times that of the main receiver) control pulses cannot reach the blanker switch in time to prevent the noise from entering the receiver IF. A large "lump-element" filter would have to be used to compensate for a delay of this magnitude. However, since the current trend is toward radio size reduction, the size of such a filter would be prohibitive.
Tuned circuits have also been used to provide this delay, but the manufacturing labor used to tune such a circuit would also make such a filter less advantageous.
Moreover, the bandwidth of the noise blanker cannot be extended to be comparable to the bandwidth of the main receiver because much more other carriers or strong information signals may be interpreted as noise and would "blank" the receiver. Overall, the effect of such a noise blanker bandwidth extension would be that the main receiver would be inhibited most of the time.
A solution to the delay problem was achieved using a surface acoustic wave (SAW) filter to afford both selectivity and time delay in an appropriately sized filter as disclosed in U.S. Pat. No. 4,654,885, titled "Mobile Radio Range Extender with Saw Filter." The SAW filter can provide a sufficient delay, does not cause significant pulse stretching, and provides a narrow IF bandwidth.
Such a SAW filter can blank certain off or adjacent carriers (carriers having frequencies close to the desired carrier frequency) but the SAW filter alone will not prevent splatter from these off-channel carriers. In the presence of ignition noise then, splatter is caused by any strong signal components passing through the SAW filter and arriving at the blanker switch.
Splatter is the generation of on-channel energy created by blanking off-channel carriers. All carriers that fall into the passband of the pre-selector and of whatever other selectivity there is before the blanker switch are switched OFF (or blanked out) by the blanker switch. However, each time blanking pulses occur, a spectrum of side band energy is created from each carrier mixing with the blanking pulses applied to the blanker switch. If these sidebands fall into the IF frequency range they may pass through the radio crystal filter as unwanted signals. Thus, this process of picking up such unwanted signals creates splatter. Hence, splatter depends on the frequency spectrum of the blanking pulses and the amplitude of the off-channel carriers and their frequency separation to the receiver or operating channel. The frequency spectrum of the blanking pulses, in turn, depend on the pulse rate and the pulse shape.
If splatter is not prevented by a splatter control circuit, the receiver sensitivity, as measured by its signal-to-noise ratio, can degrade by up to 50 dB compared to a radio without a noise blanker, or one with the blanker circuitry disabled. Thus a need exists to provide effective splatter control during noise blanking while contemporaneously providing broad receiver bandwidth and radio size reduction.